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Hi Matt,<br>
<br>
Thanks for taking time and giving so thorough explanation and a
suggestion. Much appreciated. :-)<br>
<br>
I figured out some of this, except for the last, main part. I
thought the problem was in not being able to instantiate MonadError
AppException IO, not that ExceptT doesn't have MonadUnliftIO. <br>
<br>
I'm refactoring some db code and was just exploring possible
short-circuiting ways in SqlPersistT with custom exception types,
like get404. <br>
Which approach would you suggest for that?<br>
<br>
<br>
<br>
vlatko<br>
<br>
<blockquote type="cite"
cite="mid:CA+4qU94_fgEM4O-=-fSVsPD_f8VVSNKdpUhY7E_h_XfcmTxYHg@mail.gmail.com"
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<div class="moz-cite-prefix">-------- Original Message --------<br>
Subject: Re: [Haskell-cafe] SqlPersistT action in MonadError
issue<br>
From: Matt <a class="moz-txt-link-rfc2396E"
href="mailto:parsonsmatt@gmail.com"><parsonsmatt@gmail.com></a><br>
To: <a class="moz-txt-link-abbreviated"
href="mailto:vlatko.basic@gmail.com">vlatko.basic@gmail.com</a><br>
Cc: haskell-cafe <a class="moz-txt-link-rfc2396E"
href="mailto:Haskell-cafe@haskell.org"><Haskell-cafe@haskell.org></a><br>
Date: 14/07/18 20:09<br>
</div>
<br>
<br>
</span>
<meta http-equiv="content-type" content="text/html; charset=utf-8">
<div dir="ltr">The type of `runPool` is given here: <a
href="https://www.stackage.org/haddock/lts-12.0/persistent-2.8.2/Database-Persist-Class.html#v:runPool"
moz-do-not-send="true">https://www.stackage.org/haddock/lts-12.0/persistent-2.8.2/Database-Persist-Class.html#v:runPool</a>
<div><br>
</div>
<div> <a class="gmail-def" id="gmail-v:runPool"
style="margin:0px;padding:0px;text-decoration:none;font-weight:bold;background-color:rgb(255,255,0);color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left"
moz-do-not-send="true">runPool</a><span
style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><span> </span>::<span> </span></span><a
href="https://www.stackage.org/haddock/lts-12.0/conduit-1.3.0.3/Conduit.html#t:MonadUnliftIO"
title="Conduit"
style="margin:0px;padding:0px;text-decoration:none;font-weight:bold;color:rgb(171,105,84);font-family:monospace;font-size:14px;text-align:left"
moz-do-not-send="true">MonadUnliftIO</a><span
style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><span> </span>m
=> c -><span> </span></span><a
href="https://www.stackage.org/haddock/lts-12.0/persistent-2.8.2/Database-Persist-Class.html#t:PersistConfigBackend"
title="Database.Persist.Class"
style="margin:0px;padding:0px;text-decoration:none;font-weight:bold;color:rgb(171,105,84);font-family:monospace;font-size:14px;text-align:left"
moz-do-not-send="true">PersistConfigBackend</a><span
style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><span> </span>c
m a -><span> </span></span><a
href="https://www.stackage.org/haddock/lts-12.0/persistent-2.8.2/Database-Persist-Class.html#t:PersistConfigPool"
title="Database.Persist.Class"
style="margin:0px;padding:0px;text-decoration:none;font-weight:bold;color:rgb(171,105,84);font-family:monospace;font-size:14px;text-align:left"
moz-do-not-send="true">PersistConfigPool</a><span
style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><span> </span>c
-> m a</span></div>
<div><span
style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br>
</span></div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">The
return type of `runPool`, then, is `m a`, for some `m`
satisfying `MonadUnliftIO`.</span><br>
</div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br>
</span></div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">The
type of `liftIO` is `liftIO :: (MonadIO m) => IO a ->
m a`. This means that the first argument to `liftIO` must be
an `IO a`.</span></div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br>
</span></div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">When
we say `liftIO (runPool ...)`, GHC tries to unify the `m a`
from `runPool` and the `IO a` from `liftIO`. It is able to
do this, as `IO` is an instance of `MonadUnliftIO`. Then,
the concrete type of `runPool pgConf f1 pgPool` becomes `IO
Bool`.</span></div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br>
</span></div>
<div><span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">GHC
now tries to unify the `m` in `f1 :: (MonadIO m, MonadError
AppException m) => SqlPersistT m Bool` with `IO`. It
tries to satisfy the constraints: `MonadIO` is easily
satisfied, but `MonadError AppException m` triggers the
problem you mention.</span></div>
<div><br>
</div>
<div>Because `ExceptT` does not have an instance for
`MonadUnliftIO`, we cannot use it with the `m` in `SqlPersistT
m Bool` directly. We can, instead, use `mapReaderT` to push
the `Either` into the return type, like this:</div>
<div><br>
</div>
<div><font face="monospace, monospace">pushException ::
SqlPersistT (ExceptT e m) a -> SqlPersistT m (Either e a)</font></div>
<div><font face="monospace, monospace">pushException =
mapReaderT runExceptT</font></div>
<div><br>
</div>
<div><br>
</div>
<div>Now, we can write:</div>
<div><br>
</div>
<div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial"><font
face="monospace, monospace">liftIO $ runPool pgConf
(pushException f1) pgPool</font></div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial"><br>
</div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial">This
gives us an `IO (Either AppException Bool)`, which, after
lifting, gives us `AppM IO (Either AppException Bool)`. You
can then use `either throwError pure` to pull the
`AppException` into the `ExceptT` again.</div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial"><br>
</div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial">---</div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial"><br>
</div>
<div
style="font-size:small;text-decoration-style:initial;text-decoration-color:initial">I
would suggest that you reconsider this approach, however.
The `persistent` library uses transactions and exceptions in
a way that is *almost always* what you want, but can be
surprising, and using `ExceptT` will break this system.
Transactions are automatically rolled back on a
`MonadCatch`-style exception, but they are not automatically
rolled back on an `ExceptT`-style exception. Having a single
`AppException` type that represents errors that can occur in
database transactions *and* the rest of your application is
also going to be a cause for unsafety and errors, as the
type cannot possibly be precise enough to provide any
safety.</div>
<br>
</div>
</div>
<div class="gmail_extra"><br clear="all">
<div>
<div class="gmail_signature" data-smartmail="gmail_signature">
<div dir="ltr">
<div>Matt Parsons</div>
</div>
</div>
</div>
<br>
<div class="gmail_quote">On Sat, Jul 14, 2018 at 10:22 AM,
Vlatko Basic <span dir="ltr"><<a
href="mailto:vlatko.basic@gmail.com" target="_blank"
moz-do-not-send="true">vlatko.basic@gmail.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div
style="background-color:rgb(255,255,255);color:rgb(0,0,0)"
text="#000000" bgcolor="#FFFFFF">
<p>Hello,</p>
<p>I'm trying to use SqlPersistT funcs with MonadError,
but am failing in writing the runDB correctly. <br>
</p>
<p>I tried several things, but always getting:</p>
<p> • Couldn't match type ‘IOException’ with
‘AppException’<br>
arising from a functional dependency between:<br>
constraint ‘MonadError AppException IO’
arising from a use of ‘f1’<br>
instance ‘MonadError IOException IO’ at <no
location info><br>
• In the second argument of ‘runPool’, namely ‘f1’<br>
In the second argument of ‘($)’, namely ‘runPool
pgConf f1 pgPool’<br>
In a stmt of a 'do' block: liftIO $ runPool pgConf
f1 pgPool</p>
I understand there already is "instance MonadError
IOException IO" and fundep says it can be only one for IO.
<p>How to make it compile?</p>
<br>
<p>Best regards,</p>
<p>vlatko</p>
<p> <br>
</p>
<p>Here is the minimal reproducible code:<br>
</p>
<font size="-1"><tt>module Test where</tt><tt><br>
</tt><tt><br>
</tt><tt>import Prelude</tt><tt><br>
</tt><tt>import Control.Exception.Base</tt><tt><br>
</tt><tt>import Control.Monad.Except</tt><tt><br>
</tt><tt>import Control.Monad.Trans.Reader</tt><tt><br>
</tt><tt>import Database.Persist.Postgresql</tt><tt><br>
</tt><tt><br>
</tt><tt>data AppException =</tt><tt><br>
</tt><tt> ExcText String</tt><tt><br>
</tt><tt> | ExcIO IOException</tt><tt><br>
</tt><tt> deriving (Show)</tt><tt><br>
</tt><tt><br>
</tt><tt>type AppM m = ExceptT AppException (ReaderT App
m)</tt><tt><br>
</tt><tt>data App = App { pgConf :: PostgresConf,
pgPool :: ConnectionPool}</tt><tt><br>
</tt><tt><br>
</tt><tt>runDB :: (MonadIO m, MonadError AppException m)
=> AppM m Bool<br>
</tt><tt>runDB = do</tt><tt><br>
</tt><tt> App{..} <- lift ask</tt><tt><br>
</tt><tt> liftIO $ runPool pgConf <b>f1</b> pgPool
-- error</tt><tt><br>
</tt><tt> -- liftIO $ runPool pgConf <b>f2</b> pgPool
-- OK</tt><tt><br>
</tt><tt><br>
</tt><tt><br>
</tt><tt>f1 :: (MonadIO m, MonadError AppException m)
=> SqlPersistT m Bool</tt><tt><br>
</tt><tt>f1 = throwError $ ExcText "exception"</tt><tt><br>
</tt><tt><br>
</tt><tt>f2 :: MonadIO m => SqlPersistT m Bool</tt><tt><br>
</tt><tt>f2 = return True</tt></font><br>
<br>
</div>
<br>
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